1. Technical Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to phase-locked loops used in circuitry of wireless communication devices.
2. Description of Related Art
Mobile communication has changed the way people communicate and mobile phones have been transformed from a luxury item to an essential part of every day life. The use of mobile phones today is generally dictated by social situations, rather than being hampered by location or technology. While voice connections fulfill the basic need to communicate, and mobile voice connections continue to filter even further into the fabric of every day life, the mobile Internet and moving video, including broadcast video, are the next steps in the mobile communication revolution. The mobile Internet is poised to become a common source of everyday information, and easy, versatile mobile access to this data will be taken for granted. Similarly, video transmissions to handheld user equipment will allow movies and television programs to be viewed on the go.
Third generation (3G) cellular networks have been specifically designed to fulfill many, if not all, of these future demands. As these services grow in popularity and usage, factors such as cost efficient optimization of network capacity and quality of service (QoS) will become even more essential to cellular operators than it is today. These factors may be achieved with careful network planning and operation, improvements in transmission methods, and advances in receiver techniques. To this end, carriers want technologies that will allow them to increase downlink throughput and, in turn, offer advanced QoS capabilities and speeds that rival those delivered by cable modem and/or DSL service providers. In this regard, networks based on Code Division Multiple Access (CDMA) technology or Wideband Code Division Multiple Access (WCDMA) technology may make the delivery of data to end users a more feasible option for today's wireless carriers.
The General Packet Radio Service (GPRS) and Enhanced Data rates for GSM (EDGE) technologies may be utilized for enhancing the data throughput of present second generation (2G) systems such as GSM. The Global System for Mobile telecommunications (GSM) technology may support data rates of up to 14.4 kilobits per second (Kbps), while the GPRS technology may support data rates of up to 115 Kbps by allowing up to 8 data time slots per time division multiple access (TDMA) frame. The GSM technology, by contrast, may allow one data time slot per TDMA frame. The EDGE technology may support data rates of up to 384 Kbps. The EDGE technology may utilizes 8 phase shift keying (8-PSK) modulation for providing higher data rates than those that may be achieved by GPRS technology. The GPRS and EDGE technologies may be referred to as “2.5G” technologies.
The Universal Mobile Telecommunications System (UMTS) technology with theoretical data rates as high as 2 Mbps, is an adaptation of the WCDMA 3G system by GSM. One reason for the high data rates that may be achieved by UMTS technology stems from the 5 MHz WCDMA channel bandwidths versus the 200 KHz GSM channel bandwidths. The High Speed Downlink Packet Access (HSDPA) technology is an Internet protocol (IP) based service, oriented for data communications, which adapts WCDMA to support data transfer rates on the order of 10 megabits per second (Mbits/s). Developed by the 3G Partnership Project (3GPP) group, the HSDPA technology achieves higher data rates through a plurality of methods.
Where HSDPA is a downlink protocol, High Speed Uplink Packet Access (HSUPA) technology addresses the uplink communication. HSUPA is also specified by the 3GPP group to provide a complement data link to HSDPA. HSUPA also offers broadband IP and is based on software. HSUPA also extends the WCDMA bit rates, but the uplink rates may be less than the downlink rates of HSDPA. Where prior protocols severely limited the uplink connections, HSUPA allows for much higher uplink rates.
Likewise, standards for Digital Terrestrial Television Broadcasting (DTTB) provide for transmission of broadcast video. Three leading DTTB systems are the Advanced Television Systems Committee (ATSC) system, the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) system, and the Digital Video Broadcasting (DVB) system, which includes terrestrial transmission under Digital Video Broadcasting-Terrestrial (DVB-T) specifications and transmissions to handheld devices under Digital Video Broadcasting-Handheld (DVB-H) specifications. DVB-H is an adaptation of DVB-T to handheld units, in which additional features are implemented to meet specific requirements of handheld units. DVB-H allows downlink channels with high data rates and may be made as enhancements to current mobile wireless networks. DVB-H may use time slicing technology to reduce power consumption of handheld devices.
In order to practice the various protocols, a wireless communication device is utilized. For a wireless communication device to participate in wireless communications, it typically includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). The transmitter typically includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with a local oscillator signal to produce radio frequency (RF) signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
The receiver is coupled to an antenna and typically includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with a local oscillator signal to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
One of the components in a typical RF front end is the local oscillator that generates a local oscillator signal that is sent to a mixer to mix the inbound RF signal for down-conversion and/or to mix the outbound signal in the transmitter for up-conversion to a RF signal for transmission. A common technique is to utilize a phase-locked loop (PLL) circuitry for generating the local oscillator signal. PLL circuits implement a closed loop phase or frequency control system that looks at the input and output of the PLL and uses a difference signal to correct for variations at the output. In this manner, a PLL may provide a stable local oscillator signal at its output.
When a wireless communication device includes both a transmitter and a receiver resident thereon, separate RF mixers are used on the receive side and the transmit side. The local oscillator (LO), such as the above-mentioned PLL, provides the LO signal to the mixers for up-conversion or down-conversion. With both a transmitter and a receiver, one practice is to use a single PLL that generates LO signal(s) for both transmit and receive frequency conversion. Disadvantage are noted for a single PLL generating both receive and transmit LO signals. For example, the use of one reference frequency (e.g. crystal reference) limits the available frequencies for both the transmit and receive sides. Where transmit and receive LO frequencies are the same or close to each other, a single PLL may suffice. However, if the transmit and receive LO signals are significantly apart, then it may be difficult to obtain both frequencies from a single PLL source. Additionally, if multiple frequencies are being generated from a single PLL, there may be limitations placed on the switching speed when frequencies are to be changed.
In order to circumvent the disadvantages imposed by a single PLL design, another technique uses two separate PLLs; one for the transmit side and the second for the receive side. A transmit PLL provides a transmit LO signal to the transmitter mixer and a receive PLL provides a receive LO signal to the receiver mixer. In this manner separate LO frequencies may be generated for the receiver and the transmitter. However, with the use of two PLLs, there is still the problem of the first PLL not tracking with the second PLL or, alternatively, where the second PLL is dependent on the first, independent operation (such as frequency selection or switching) between the two PLLs is difficult to achieve.
For example, when multiple RF frequencies are to be converted in the receiver mixer or generated in the transmitter mixer, the LO frequencies from the PLL most likely needs to switch rapidly. Thus, for higher frequency devices, PLL switching speed should not be a limitation. Prior art PLL circuitry may not have the capability to perform the switching operation and maintain the stability in the LO signals. Further, when transmitters and receivers have different frequency requirements, independence in selection of receive side LO and transmitter side LO is useful.
Therefore, a need exists for a technique to implement a robust PLL circuitry for use as a local oscillator to generate both transmit and receive LO signals that are independent of each other, but still operate together to track variations in the reference input.